In the field of semiconductor integrated circuits a large number of identical circuits are commonly manufactured using a single sheet of semiconductor material known in the art as a wafer. The wafer is then diced to separate the individual integrated circuits. Each integrated circuit thus separated is known as a die or a chip.
The distribution of defects on a wafer is reasonably predictable. Commonly any one defect on a die will prevent that die from functioning properly. Therefore, a predictable number of defective dice will be produced on each wafer. Therefore, a common goal in the production of semiconductor integrated circuits is to reduce the number of defects or otherwise improve the yield of properly functioning circuits. Various techniques are used to accomplish this.
One technique, which is particularly effective in semiconductor dice involving what is known as a pad limited design, is the use of redundancy. A pad limited design is one in which the positioning of electrical contact pads on the surface of the die in order to facilitate electrical contact with other circuitry requires the use of a larger die than would be required simply for the circuitry in the chip. Thus, additional circuit elements may be provided without increasing the die's size. If the additional circuit elements thus provided are identical to circuit elements already present in the basic design, the die is said to be redundant with regard to those circuit elements. Typically, when redundant circuitry is used, each die must be tested to determine if there is a defect in any of the redundant circuits. When one of the circuit elements is found to be good it is connected to the remainder of the circuitry by means of electrical conductors, typically deposited on the surface of the die.
This prior art method requires testing of the circuit elements in order to determine whether a particular element is good. Further it requires the additional step of depositing conductors on the surface to complete the circuit and connect the good circuit elements into the circuit. A preferrable system would provide redundancy without the requirement of testing and preferential wiring.